The present invention relates to power transmission devices including: a shift device that is capable of transmitting power applied to an input shaft to an output shaft by shifting a speed ratio to a plurality of shift speeds by engagement and disengagement of a plurality of hydraulic friction engagement elements; a differential mechanism that is coupled to right and left driving wheels; a countershaft that transmits the power from the output shaft to the differential mechanism; and a case that accommodates the shift device, the differential mechanism, and the countershaft.